This invention relates to an improved method and system for the vertical continuous, or seim-continuous, casting of molten metal to provide solidified ingot thereof.
More particularly, the invention relates to a method and system for distributing molten metal in a vertical open ended mold of the type often designated a direct-chill mold and including a molten metal distributor arrangement which floats in the molten metal within the mold and includes a variable weighted system which facilitates control of the molten metal level or head in the casting mold.
The improvement is particularly suited to the direct-chill casting of light metals, specifically aluminum, magnesium, and their alloys.
In general, a process of direct chill continuous casting of metals includes feeding a molten metal into a vertical open-ended mold and continuously withdrawing a solid casting from the lower end of the mold. At the initiation of the casting operation when molten metal is fed into the mold, the lower end is closed by a downward movable bottom die. A supply of cooling fluid is continuously applied to the mold and also to the solid ingot surface as the ingot emerges from the lower end of the mold as the movable bottom die moves downwardly. It is desirable in the process to distribute the molten metal to provide a homogeneous solid casting having a relatively smooth surface free from thermal cracks, cold shuts and other defects. Such defects can introduce problems in later fabricating operations such as rolling and can add to the costs of fabrication by necessitating a machining operation to remove a surface portion of the casting having the defects. It is therefore desirable to reduce or eliminate any defects from the outer surface of a continuous cast ingot.
It is known in the art to provide a distributor, generally disposed beneath a feed spout, for laterally spreading molten metal flowing from a spout to uniformly distribute the molten metal and its heat content to the sides of the mold. A distributor may have a float portion which allows the float-distributor to float in the molten metal and rise and fall within the mold as the level of molten metal changes in the mold. In such manner, the float-distributor operates as a valve to control metal flow rate through the spout by engaging against the lower end of the feed spout when the molten metal flow rate is too great, and the molten metal head (i.e. height of molten metal in the mold) is too high. Similarly, when the molten metal level is too low in the mold, the float-distributor is lowered away from the lower end of the spout to permit more molten metal to enter the mold. Thus, the float-distributor acts as a leveling control to maintain a constant molten metal head throughout the casting operation. U.S. Pat. No. 2,876,509 granted Mar. 10, 1959, U.S. Pat. No. 3,672,431 granted June 27, 1972, and U.S. Pat. No. 3,752,217 granted Aug. 14, 1973, illustrate various configurations of the prior art to distribute molten metal and to maintain a constant level of molten metal in the mold.
It is further known in the art that a lower head of molten metal in the mold improves the surface characteristics of the ingot produced. One reason for such an improved surface is that there is less contact between the solidifying molten metal and the mold before it emerges from the lower end of the mold. At the initiation of the continuous casting operation, however, if the distance between the float-distributor and the movable bottom die of the mold is too small, the float-distributor tends to freeze in the solidifying metal. That problem can be solved by providing a higher head of molten metal at initiation of the casting operation, and changing to a lower head after casting is begun. U.S. Pat. No. 3,425,482 granted Feb. 4, 1969, discloses raising the mold or lowering the tundish or feed spout into the mold to lower the head of molten metal in the mold ater the casting operation is initiated. Such an arrangement can add to the complexity of the continuous casting operation and can be costly to install and operate. Furthermore, operating a substantial number of molds, typically 6 to over 20, at the same time as is commercial practice further increases the costs and complexity of operation. Therefore, it is desirable to provide an uncomplicated and inexpensive system that can be readily incorporated into existing direct chill continuous casting arrangements to lower the molten metal head in the mold during casting to improve the surface quality of a continuous cast ingot.